a) Field of the Invention
The present invention relates to a nanocrystalline composite for use to store hydrogen.
The invention also relates to methods of preparing such composite and to their use.
b) Brief Description of the Prior Art
The main reasons to develop and use a composite are essentially (1) to take advantage of the unique properties of each component of the composite and (2) to profit from its complex (multi-component) microstructure.
In the case of a composite for use to store hydrogen, the purpose of combining two or more hydrogen carriers is essentially to be able to modify the hydrogenation/dehydrogenation properties of the resulting composite in such a way as to provide a wider range of operational conditions.
Although a large variety of hydrogen carriers exists, which mainly consist of metal hydrides operating at temperatures ranging between -40.degree. C. and 500.degree. C. there is no suitable hydrogen carrier which provides optimum hydrogenation conditions along with a high hydrogen storage capacity. By way of examples, in the case of a hydrogen-fueled vehicle, such "optimum" hydrogenation conditions would be an ability to absorb/desorb hydrogen at a temperature of about 150.degree. C. while a "high" hydrogen storage capacity would be an ability for the carrier to store more than 3% by weight of hydrogen.
So far, there is no hydrogen carrier which can meet both of these requirements. Indeed, all the hydrogen carriers which can operate at temperatures lower than 100.degree. C. have a hydrogen storage capacity by weight that is too low to be effective in transportation. For example, FeTi has a storage capacity of 1.9 wt % while LaNi.sub.5 has a capacity of 1.3 wt %. On the other hand, all the hydrogen carriers which exhibit a high hydrogen storage capacity, such as, for example Mg.sub.2 Ni which has a capacity of 3.6 wt % or Mg which has a capacity of 7.65 wt %, require temperatures higher than 300.degree. C. for hydrogenation/dehydrogenation cycling. Of course, the need for high temperature (usually ranging from 300 to 400.degree. C. for absorption/desorption decreases the efficiency of such carriers and the potential development and use of vehicles using hydrogen as a fuel.
In copending U.S. patent application Ser. Nos. 08/382,776 and 387,457 filed on Feb. 2 and 13, 1995, respectively in the name of the same assignees, a new generation of hydrogen carriers is disclosed, which consist of nanocrystalline metal hydride powders incorporating or not a catalyst.
More particularly, U.S. application Ser. No. 08/387,457 discloses a powder of an alloy of Ni and Mg, La, Be or Ti, consisting of crystallites having a grain size lower than 100 nm and preferably lower than 30 nm and a crystalline structure allowing hydrogen absorption. This powder is preferably obtained by mechanical grinding and may consist of crystallites of Mg.sub.2 Ni, LaNi.sub.5 or of Ni-based alloys of Be or Li. It is particularly useful for storing and transporting hydrogen, since it requires no or only one single activation treatment at low temperature to absorb hydrogen and its kinetic of absorption and diffusion of hydrogen is fast.
U.S. application Ser. No. 08/382,776 discloses a very light-weight, Mg and Be-based material which has the ability to reversibly store hydrogen with very good kinetics. This material is of the formula: EQU (M.sub.1-x A.sub.x)D.sub.y
wherein:
M is Mg, Be or a combination of them; PA1 A is an element selected from the group consisting of Li, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, In, Sn, O, Si, B, C and F PA1 D is a metal selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Ir and Pt (preferably Pd); PA1 x is a number ranging from 0 to 0.3; and PA1 y is a number ranging from 0 to 0.15. PA1 the problems of poisoning by oxidation; PA1 the need for activation; and PA1 too slow kinetics of hydrogenation/dehydrogenation. PA1 a) at least one first hydrogen carrier hereinafter called "high temperature metal hydride", which has a high hydrogen storage capacity but requires high temperatures for hydrogen absorption and desorption; and PA1 b) at least one second hydrogen carrier hereinafter called "low temperature metal hydride", which has a low hydrogen storage capacity but does not require high temperatures for hydrogen absorption and desorption.
This material is in the form of a powder of particles of the formula M.sub.1-x A.sub.x as defined hereinabove, each particle consisting of nanocrystalline grains having an average size of 3 to 100 nm or having a nanolayered structure with a layer spacing of 3 to 100 nm. Some of these particles have clusters of metal D attached thereto, with an average size ranging from 2 to 200 nm.
The nanocrystalline powders disclosed in both of the above U.S. applications overcome most of the drawbacks of the conventional hydrides, including:
The latter point is essential since outstanding kinetics of hydrogen uptake permits to decrease significantly the effective operational temperature of, for example, the Mg-based hydrides, to a range inaccessible for conventional, polycrystalline materials (for example 200 to 250.degree. C.).
In spite of the above advantages, many potential applications for hydrogen carriers, especially, metal hydrides, require a cold star-up of the hydrogen-fueled devices, which means ambient temperature for the initiation of the process, with the possibility of gradually switching to higher temperatures as the device warms up.